As a rotary encoder for detecting a rotation angle of a motor, there are an axial direction optical path type one in which a flat, plate-shaped scale is fitted to a rotating shaft and an optical path between a light source and a light detector is set to the scale in the same direction as the rotating shaft, and a radial direction optical path type one in which a so-called drum type scale formed in a generally cylindrical shape with a slit formed in a side surface portion thereof is fitted to the rotating shaft and the optical path is set in the radial direction.
For example, in a compact motor for use in a household printer and the like, a volume needed to install a rotary encoder on the compact motor is desired to be made as small as possible. In such an application, also as shown in Patent Document 1 and the like, there is an advantage in a radial direction optical path type rotary encoder of having a benefit that an entire volume can be made small with ease.
However, in the radial direction optical path type rotary encoder, since the scale is generally cylindrically shaped with a slit formed in a side surface portion thereof, it is difficult to maintain accuracy compared to a case of forming a slit in a flat, plate-shaped scale and it is difficult to improve a resolution.
In more specific explanation, for example, in a case where a drum type scale is entirely formed of a resin by injection-molding, it is necessary to form the slits by arranging a plurality of thin column-shaped light blocking portions and light transmitting portions without anything formed and through which light passes as it is, alternately in a circumferential direction so that the side surface portion is comb tooth shaped. For example, in the case where burrs are generated in this light blocking portion at the time of molding, a quantity of light allowed to pass through the light transmitting portion is varied from a designed value or a quantity of light passable through each of the light transmitting portions is varied and this results in arising a problem that performance of the rotary encoder is hard to be stabilized in individual products.
Further, a die used for molding the resin scale is deteriorated because an edge portion in the vicinity that forms the light blocking portion or light transmitting portion becomes dull or the like when the injection-molding is repeated. In the case where deterioration such as dulling of the die occurs, new burrs which were not generated at an initial time of production are prone to generation and the light blocking portion is formed larger than the designed one, and therefore a quantity of light allowed to pass through the light transmitting portion is deviated from a designed value, which results in it being more difficult to maintain the performance such as a resolution.
In this way, in the case of drum type scales, since their shape is more complicated compared to a case of a flat, plate-shaped scale problems often occur such as burrs and the difficulty in maintaining accuracy due to deterioration of the die as mentioned above, and as a result problems occur such as the variation of performance of individual rotary encoders and difficulties with improvement of resolution of the rotary encoder itself.